1. Field of the Invention
The present invention relates to a control device for a diesel engine, which advances a fuel injection timing to be earlier than in diesel combustion, and carries out the Premixed Compression Ignition that injects fuel in a plurality of stages.
2. Description of the Related Art
Generally in the diesel combustion (diffusion combustion), air is taken into cylinders to be compressed. Fuel is then injected into the compressed air to be combusted by spontaneous ignition. Aside from this method, there is another combustion method, namely Homogeneous Charge Compression Ignition (HCCI) combustion, which advances the fuel injection timing considerably to elongate an ignition delay period. This method produces a lean mixture and combusts the mixture at low temperature, thereby suppressing the generation of NOx and smoke.
In the process of the HCCI combustion, when the injection quantity grows as the demanded load is increased, the fuel is pre-ignited, and combustion is intensified, which causes knocking. There also arouses the fear that a large quantity of NOx and smoke might be produced. To solve these troubles, there is a premixed compression ignition method that advances the fuel injection timing to be much earlier than in diesel combustion, but injects fuel in a plurality of stages. For example, some of the total fuel quantity is injected in a plurality of stages with early timing in a compression stroke, and the rest is injected near the compression top dead center.
The fuel injected with the early timing in the compression stroke, however, reaches as far as a cylinder wall surface, which triggers oil dilution and smoke. This is because a piston is still located in a low position at this point, and the atmosphere density (gas density) in the corresponding cylinder is low. In this light, there are disclosed the technology of injecting low-pressure fuel so that the fuel is prevented from reaching the cylinder wall surfaces (Unexamined Japanese Patent Publication No. 2002-201991) and the technology created in light of the gas density in the corresponding cylinder in relation to the fuel injected with the early timing (Unexamined Japanese Patent Publication No. 2003-286879).
According to the technology disclosed in Publication No. 2002-201991, an injection pressure is controlled by means of a fuel injector having two kinds of common rails, that is, low-pressure and high-pressure common rails. If the injection is performed with respect to fuel supplied from the low-pressure common rail in a plurality of stages, the premixed compression ignition method can be actualized.
However, the penetration of fuel spray is successively increased or decreased according to atmosphere in the cylinder. Unless the injection pressure is changed according to the state in the cylinder at the time of injection, it is impossible to actually prevent the fuel from reaching the cylinder wall surface. In other words, when the fuel is injected by stages, there is supposed to be the optimum injection pressure for each injection. According to the technology disclosed in Publication No. 2002-201991, however, the injection pressures of their respective stages are all set to equal pressures. This causes the problem that especially the fuel injected at the first stage is liable to reach the cylinder wall surface. Evaluation of the quantity of the fuel that adheres to the wall surface in the above-described technology relates to post injection, and is not applied with main injection taken into account. Such evaluation is hard to apply in case that all fuel is combusted in the inside of cylinders.
There is one idea, as shown in the technology disclosed in Publication No. 2003-286879, that the technology of controlling an injection pressure and the like in consideration of gas density in the corresponding cylinder is combined with the technology described in Publication No. 2002-201991. However, without consideration of atmosphere temperature (gas temperature) in the cylinder in addition to the gas density, the fuel spray penetration does not follow actual conditions, and the quantity of the fuel that actually adheres to the cylinder wall surface cannot be evaluated. This is because an increase in capacity of the cylinder not only reduces the gas density but also lowers the gas temperature, and the fuel spray penetration in such a case is on the increase, which further enhances the possibility that the fuel reaches the cylinder wall surface. Another reason is that a rise in the intake temperature reduces the gas density but increases the gas temperature, so that the fuel spray penetration in this case is on the decline due to fuel evaporation, which decreases the possibility that the fuel reaches the cylinder wall surface.